Datasheet

V
SNS
= 1.24V x
R
CSH
R
HSP
I
CSH
=
R
HSP
V
SNS
R
T
LM3424
RT
Start t
ON
Oscillator
R
FLT
C
AC
C
FLT
External Synchronization
PWM
f
SW
=
1.40e
-10
x R
T
- 1.95e
-8
1
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
PEAK CURRENT MODE CONTROL
Peak current mode control is used by the LM3424 to regulate the average LED current through an array of
HBLEDs. This method of control uses a series resistor in the LED path to sense LED current and can use either
a series resistor in the MosFET path or the MosFET R
DS-ON
for both cycle-by-cycle current limit and input voltage
feed forward. The controller has a fixed switching frequency set by an internal programmable oscillator which
means current mode instability can occur at duty cycles higher than 50%. To mitigate this standard problem, an
aritifical ramp is added to the control signal internally. The slope of this ramp is programmable to allow for a
wider range of component choices for a given design. A detailed explanation of this control method is presented
in the following sections.
SWITCHING FREQUENCY
The switching frequency of the LM3424 is programmed using an external resistor (R
T
) connected from the RT pin
to GND as shown in Figure 20.
Alternatively, an external PWM signal can be applied to the RT pin through a filter (R
FLT
and C
FLT
) and an AC
coupling capacitor (C
AC
) to synchronize the part to an external clock as shown in Figure 20. If the external PWM
signal is applied at a frequency higher than the base frequency set by the R
T
resistor, the internal oscillator is
bypassed and the switching frequency becomes the synchronized frequency. The external synchronization signal
should have a pulse width of 100ns, an amplitude between 3V and 6V, and be AC coupled to the RT pin with a
ceramic capacitor (C
AC
= 100pF). A 10MHz RC filter (R
FLT
= 150Ω and C
FLT
= 100 pF) should be placed between
the PWM signal and C
AC
to eliminate unwanted high frequency noise from coupling into the RT pin.
The switching frequency is defined:
(4)
See the Typical Performance Characteristics section for a plot of R
T
vs. f
SW
.
Figure 20. Timing Circuitry
AVERAGE LED CURRENT
To first understand how the LM3424 regulates LED current, the thermal foldback functionality will be ignored.
Figure 21 shows the physical implementation of the LED current sense circuitry assuming the thermal foldback
circuitry is a simple current source which, for now, will be set to zero (I
TF
= 0A). The LM3424 uses an external
current sense resistor (R
SNS
) placed in series with the LED load to convert the LED current (I
LED
) into a voltage
(V
SNS
). The HSP and HSN pins are the inputs to the high-side sense amplifier which are forced to be equal
potential (V
HSP
=V
HSN
) through negative feedback. Because of this, the V
SNS
voltage is forced across R
HSP
which
generates a current that is summed with the thermal foldback current (I
TF
) to generate the signal current (I
CSH
)
which flows out of the CSH pin and through the R
CSH
resistor. The error amplifier will regulate the CSH pin to
1.24V and assuming I
TF
= 0A, I
CSH
can be calculated:
(5)
This means V
SNS
will be regulated as follows:
(6)
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